The invention relates to integrated circuits, and more particularly, to high-density interconnection of temperature sensitive devices to integrated circuits.
The semiconductor industry continues to move to smaller, denser integrated circuits (IC""s) with greatly increased capabilities. The xe2x80x9csystem-on-a-chipxe2x80x9d has become a reality in recent years. In parallel with this trend to higher densities have been advances in integrated circuit packaging, enabling larger numbers of input/output connections to external circuitry in smaller areas. Integrated circuit packages such as the ball-grid array (BGA), Chip-scale package (CSP), and the C4 package (Flip-Chip) are examples of such packaging.
Along with these advances, new methods for interconnecting the IC packages to external printed circuit boards have also been developed. Flip-chip bonding has become an industry standard for high-density interconnections, and permits several hundred or more interconnections to be made simultaneously, and is often referred to as a mass-interconnection method. As the electronics industry continues to evolve, however, techniques for achieving greater numbers of interconnections and higher densities will be highly desirable.
Furthermore, many systems-on-a-chip being developed employ temperature sensitive devices or materials, such as plastics, piezoelectric ceramics, or organic molecular sensors. Other such temperature sensitive materials and devices will be apparent to those skilled in the art. These devices can be included on the chip itself, or can be xe2x80x9cbump bondedxe2x80x9d to the chip as a hybrid structure.
Temperature limitations associated with such devices preclude the use of conventional fabrication processes operating at relatively high temperatures. For example, lead-tin (PbSn) solder, used in numerous processes, has a melting point greater than 300xc2x0 C. Once heated above its temperature limits, a device may exhibit degraded performance or otherwise cease to function. Such damage is typically not reversible. The interconnections of such devices must therefore be fabricated with consideration given to temperature limitations.
For example, the interconnections of some infrared detector arrays are made with evaporated pure Indium solder, where solder xe2x80x9cbumpsxe2x80x9d are patterned using photolithography, and the interconnections are made by cold welding the detector array to the IC. However, in other applications (e.g., such as ultrasound arrays), the Indium cold-welding approach adapted from the infrared applications is not reliable. Other conventional methods for bump bonding temperature sensitive devices include the use of polymer xe2x80x9csolders,xe2x80x9d solder jetting, evaporation, stencil printing, solder extrusion, solder casting, electroless deposition, electroplating, sputtering, and flip-chip reflow techniques. Each of these conventional methods is associated with problems.
What is needed, therefore, are improved techniques for performing high-density mass-interconnection of temperature sensitive electronic devices.
One embodiment of the present invention provides a method for interconnecting a first structure (e.g., such as a piezoelectric ultrasonic transducer array) to a second structure (e.g., integrated circuit or assembly). The first structure has a number of layers or regions including a photosensitive polymer layer, a mask layer, electrical contacts which are accessible via holes in the mask layer, and a temperature sensitive material that has an upper temperature limit. The method includes etching the photosensitive polymer layer thereby opening a set of holes in the photosensitive polymer layer. Each of these opened holes is located over an electrical contact, and is generally wider than the corresponding hole in the mask layer.
The method further includes depositing an electrically conductive material layer on at least the electrical contact and walls of each hole in the photosensitive polymer layer. Note here, that the electrically conductive material layer can be deposited on the photosensitive polymer layer, as well as the electrical contact and walls of each hole in the photosensitive polymer layer so as to simplify the deposition process. The electrically conductive material has a melting point that is less than the temperature limit of the temperature sensitive material.
The method proceeds with removing the photosensitive polymer layer and excess portions of the electrically conductive material thereby leaving remaining portions of the electrically conductive material. Each remaining portion is electrically and mechanically connected to an electrical contact. The method further includes heating the remaining electrically conductive material above its melting point thereby forming electrically conducting volumes over the electrical contacts. Each electrically conducting volume is formed from the remaining portions of the electrically conductive material connected to the corresponding electrical contact.
Another embodiment of the present invention provides a method for interconnecting a first structure to a second structure. The first structure has a number of layers or regions and includes electrical contacts that are accessible via holes in a polymer layer, and a temperature sensitive material that has a temperature limit. The electrical contacts may be connected to the temperature sensitive material and/or to other parts included in the structure. The method includes depositing an electrically conductive material layer on at least the electrical contact and walls of each access hole. The electrically conductive material has a melting point that is less than the temperature limit of the temperature sensitive material.
The method further includes removing the polymer layer and excess portions of the electrically conductive material thereby leaving remaining portions of the electrically conductive material. Each remaining portion forms a saucer-like shape, where at least a portion of the saucer-like shape is electrically and mechanically connected to its corresponding electrical contact.
The method proceeds with heating remaining electrically conductive material above its melting point thereby forming electrically conducting volumes over the electrical contacts. Each electrically conducting volume is formed from the remaining portions of the electrically conductive material connected to the corresponding electrical contact. In alternative embodiments, the electrical contacts can be associated with an under-bump metallurgy layer to improve adhesion of the electrically conductive material. The method may further include aligning the electrically conducting volumes of the first structure with electrical contacts on the second structure, and then heating the conjoined assembly to a temperature sufficient to melt the electrically conducting volumes to form an electrical and mechanical bond between the structures.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.